The present invention relates to a test unit for grading battery cells.
In order to assemble battery packs employing multiple cells made of, for example, a series and/or series parallel connection of individual secondary cells, the cells should have substantially the same capacity. However, NiCd or metal hydride batteries can display different discharge characteristics from one another upon manufacturing. Thus, it is desirable to grade the cells prior to battery pack assembly to assure that the cells have substantially the same capacity, particularly when in a series interconnection, so that one cell, which may have less capacity than the other, does not disable or otherwise reduce the capacity of the entire battery pack.
In the past, in order to grade the cells, a test fixture has been employed for holding cells which have been fully charged, coupling them in series, and loading and discharging them through a resistive load. Such fixtures hold, for example, an array of one hundred cells with the operator inserting the cells into the test fixture to begin their discharge. Periodically, and typically on a two-minute basis, an operator utilizes a voltage test probe to test each cell. The probe provides an audible or visible indication that a cell has fallen below a predetermined reference voltage, such as 1.3 volts for a 1.5 volt cell. Thus, the operator must periodically manually test each of the hundred cells and, if a cell has fallen below the threshold voltage, the test probe will provide the operator with an indication that the cell has reached a lowered voltage. When a cell falls below the threshold voltage, it is then removed, which interrupts the series circuit of the test fixture, and the last most cell is repositioned in the removed cell's location to continue the series discharge circuit. As can be appreciated as more and more cells reach a diminished capacity, the shuffling and reshuffling of batteries becomes a labor intensive and inefficient process by which to grade cells.